The invention relates to the field of sigma delta modulators, and in particular to a sigma delta generator providing hysteresis control to control the transition rate of the modulator.
Audio power amplifiers of conventional design suffer from low efficiency (often <50%), and this causes these designs to generate heat that must be removed by large heat sinks, causing the physical amplifier designs to be quite large. In recent years, in order to make amplifiers that are smaller, high-efficiency designs have been introduced. The most common approach is to use “Class-D” switching amplifiers. These amplifiers work by converting the analog input signal into a 2-level output signal using a high-frequency modulation process. This 2-level signal is then fed to a power stage, which in turn feeds a passive LC filter connected to the speaker. The power stage is fed from a V+ and V− supply, and outputs the V+ voltage when the input is a “1” and the V− voltage when the input is a “0”. Since the output devices used in the power stage have no voltage across them while current is flowing, the heat produced by these devices is dramatically reduced.
Most prior-art systems employ a pulse-width modulation scheme, where the value of the input signal at a moment in time is represented by a fixed-voltage variable-width output pulse (PWM). A typical audio PWM amplifier can work at a switching frequency of between 100 KHz and 500 KHz. Higher switching frequencies will reduce distortion but also result in lower efficiency due to the extra transitions in the output waveform. Each transition takes a certain amount of energy to charge all the various node capacitances, and therefore reduces efficiency.
For typical clock rates of about 300 KHz, the distortion/noise performance of traditional class-D amplifiers is not very good.
Another modulation technique, which is less common, is to use a sigma-delta modulator that converts the analog input to a series of 1's and 0's at a higher sampling rate, typically about 64 times higher than the highest audio frequency. Such circuits are commonly used in A/D converter designs used for converting analog audio signals into a digital 1-bit stream. This technique yields better distortion characteristics than the PWM scheme, but the drawback is a significantly higher switching rate, resulting in lower efficiency.
The present invention uses the best features of both PWM (which has the lowest switching rate) and sigma-delta (which has the lowest distortion/noise).